DE2455850A1 - POWDER MIXTURE FOR THE MANUFACTURE OF ALLOY STEEL WITH A LOW OXIDIC INCLUSION CONTENT - Google Patents

Description

2455850 Maxton · Maxton ■ Langmaack patent attorneys

Alfred Maxton sr. Alfred Maxton Jr. Jürgen Langmaack

Dlplom Engineering

5 Cologne 51

Pferdmengosstrasse 50

Day: 25. H. 19.74

Applicant: Höganäs AB, Fack, Höganäs 1, Sweden

Title j Powder mixture for the production of alloy steel with a low, oxidic content Inclusions

Our reference: 827 P.

The present invention relates to metal powders for manufacture of alloy steel and especially of low alloy steel. Production can be carried out using conventional powder metallurgy Process (pressing and sintering) or carried out by means of the evolving hot forging process will.

In the first case, the metal powder is pressed into pellets in molds that come very close to the desired shape of the final product. The green compact is then subjected to a heat treatment in which the powder particles become

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sinter and the compact receives a good strength. Products made in this way have a certain Porosity, which causes low ductility and toughness. The porosity that varies from one material to another can vary considerably, but usually 7 to 20 percent by volume, can by use the above-mentioned hot forging technique can be eliminated, whereby the ductility and the toughness are considerably improved can be. The most important stage in hot forging is pressing the heated preformed compact into a shape. The result of hot forging is a full-density body.

In the manufacture of steel products from metal powder accordingly however, the latter technique has a significant impact on oxidic impurities present in the powder more damaging influence on the mechanical properties than in the case of products which are pressed in the usual way and were sintered.

Powders for the manufacture of products from such alloy steels can be divided into two fundamentally different groups. Initially, it is possible to mix different powders, each consisting of one or more, but not all, of the alloying elements of the final product.

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Second, it is possible to atomize a melt that has exactly the desired composition of the end product. In the latter case, each powder particle is therefore homogeneous alloyed with the same proportions of alloying elements as desired in the end product. However, this alloying method has some drawbacks that it has in in some cases render it directly unusable.

One of the major drawbacks is due to the oxide impurities in the alloy powders. In the most common and completely superior from an economic point of view of the atomization process, namely in the atomization of water, some of the powder particles become during the atomization process oxidized. In order to reduce the oxides formed in the process, the powder is subjected to a subsequent heating in a reduced atmosphere, that is, subjected to cracked ammonia.

The heating of the green compacts in a reduced atmosphere before thermoforming is another way to reduce remaining oxides in the powder, as the heating occurs up to a higher temperature than that which is used to glow the powder. By means of the present Engineering it is possible to produce substantially oxide-free thermoformed material provided that the steel powder contains only alloy elements whose

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Oxides can be reduced relatively easily, that is to say that they have a free energy of formation with an absolute value below 120 kcal / mol O ₂ (502 kJ / mol O ₂ ) at 100 ° C. Values of the free energy of formation for oxides of some alloying elements which are generally present in steel and according to Kubaschewsky, 0. & ■ Evan, LL, Metallurgical
Thermochemistry, London 1956 are listed in the table below:

oxide

Free energy of formation at 1000 ° C kcal / mol Og "... > · ' Kj / mol 0 _g

CuO ₂ - 37'-.-155

NiO - 57-239

CoO. - 67 -.381

MoO ₃ , - 67 -281

FeO · - 86 -360

^Cr 2 ° 3 -126 -528

MnO '-11 ^ 0 -586

V ₂ O ₃ '-12 * 8 -619

SiO ₂ -156. . -650

TiO ₂ -I65 -688

Al ₂ O ₃ -203

Oxides which have an absolute value for the free energy of formation which exceeds 120 kcal / mol O _g (5O2 kJ / mol O ₃ ) at 1000 ° C., are not or only incompletely after

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the current technology reduced. This makes elements sensitive to oxidation such as Cr Mni / iie very desirable from an economic as well as an alloy standpoint
can only be used to a very limited extent as alloying elements in atomized steel powders. This is because they form oxides during sputtering that cannot be removed during further processing.

In the end product, these oxides then cause a considerable deterioration in ductility and toughness. However, these properties are dependent on the size of the oxide inclusions, as has been established through experiments. at
a predetermined total. Oxygen content in the final product, the material exhibits considerably deteriorated mechanical properties when the oxygen is in the form
of gross impurities when compared to material in which the oxide impurities are small but more numerous. The critical size of the oxide inclusions, above which they cause a severe deterioration in the material properties, is between 20 μm and 100 μm.

The object of the present invention is therefore to avoid the above-mentioned difficulties by means of a suitable powder mixture,
+ / and,.

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The basic idea of the invention is that the metal powder component consists of a mixture of two powders, namely a first atomized pre-alloyed steel powder and a second non-atomized alloy powder which was produced by pulverizing a solidified melt. The alloying elements are distributed in such a way that elements whose oxides are easily reducible (preferably nickel, copper, molybdenum and / or cobalt) are essentially incorporated into the atomized pre-alloyed steel powder, while the oxidation-sensitive elements (preferably chromium, manganese) are incorporated into the fine powdered powders are introduced. This does not apply to carbon which is added essentially as graphite in amounts of up to 1% of the powder mixture or as an alloying element to the finely pulverized alloy powder in an amount of not more than 10% of this powder. Both here and in the following, percent is always understood to be percent by weight.

The atomized pre-alloyed powder is produced by melting iron and 0.2 to 10 $ alloy elements, the oxides of which have an absolute value for the free energy of formation below 120 kcal / mol Og (502 Rj / mol 0 _g ) at 1000 ° C, after which a Water atomization of the melt and finally an annealing in a reduced atmosphere (suitably in cracked ammonia) takes place * Depending

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WT. ^fc

Depending on the quality of the raw iron material, the atomized pre-alloyed powder can contain up to 0.2 k% of additional elements, the oxides of which have an absolute value for the free energy of formation above 120 kcal / mol Og (502 kJ / mol Og) at 1000 ° C. The proportion of such additional elements, the oxides of which have an absolute value for the free energy of formation above 150 kcal / mol O ₀ (627 kJ / mol O 2) at 1000 ° C., should not exceed 0.1 $. In particular, the proportion of silicon, titanium and aluminum should be 0.0it ^, $ 0.03 and. Do not exceed $ 0.03. The powder should have a particle size distribution such that more than <$ 0% and preferably more than 97% of the powder passes through a sieve with a mesh size of 175 μm (80 Tyler mesh method for the sieve analysis of granular metal powders,. MPIF Standard 5- J + 6, Metal Powder Industries Federation, New York, USA).

Other alloying elements, the oxides of which have an absolute value for the free energy of formation that exceeds 120 kcal / mol Og (502 kj / mol Og) at 1000 ° C, are possibly with the addition of at most 75 % _t, preferably at most 50 $ iron and / or others Metals with easily reducible oxides and carbon melted and cast. The ingot is crushed and pulverized into a fine alloy powder. The total proportion of elements whose oxides form one

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Having the absolute value of the formation free _{energy exceeding 150 kcal / mol O 2} (627 kJ / mol Og) at 1000 ° C should not be higher than 1% in the finely pulverized alloy powder.

The two components are then mixed in proportions of 80 to 99 # atomized pre-alloy powder and 20 to 1% finely pulverized alloy powder. A powder mixture produced in this way results in a material that has only small and few oxide particles, as the following example shows »

example 1

Two powders were prepared, one by atomizing it with water and then calcining it in cracked ammonia (A), the other by mixing fine iron alloy base powder and water-atomized molybdenum alloy steel powder (B). The two powders (A) ^ (B) had the composition 1% Mn, 1% Cr, 0.5 $ Mo, the remainder Fe. The iron alloy powder in powder (B) had the composition 25 $ Mn, 25 # Cr, 7 # C, remainder Fe and a mean particle size of 1 ^ un according to Fischer (method for determining the mean particle size of metal powder by the Fischer eub-sieve sizer , MPIF Standard 32-60, Metal Powder

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Industries Federation, New York, USA), In both cases < Graphite added in such an amount that the carbon content of the powder was $ 0.5.

Green compacts in the form of cylinders with a diameter of 25mm and 30mm in length were pressed from the two powders. The compacts were heated, one group to 1120 ° C and one others to 1200 ° G in a hydrogen gas atmosphere nut a dew point of minus 20 ° C and at these temperatures Held for 30min. The pellets came out of the oven quickly brought into a form in which the cylinders were pressed to full density at elevated temperatures.

In order to determine the content of oxide inclusions, the Cylinder analyzed for total oxygen content, Furthermore, the number of oxide inclusions which had a linear dimension in any direction greater than 100 µm was counted on a cross-sectional area of each cylinder. The result is shown in the following table:

Temp. Material oxygen content number of oxide

conclusions

° C % 100 yum / cm ³

1120 A - 0.54 116

B 0.20 10

1200 A 0.23 * V7

B 0.06 3

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This example shows the advantages of the present invention when Cr and Mn, the oxides of which have a free energy of formation of -126 or -li + O kcal / mol Op (-528 and -586 kj / mol O ₂ ) at 1000 ° C, are added as finely powdered iron alloy (powder B), the thermoformed product has a considerably lower oxygen content and a lower number of large inclusions than in the case where chromium and manganese were already alloyed to the atomized steel powder (powder A).

As mentioned above, in addition to having a low inclusion content, it is desirable that the alloying elements are homogeneously distributed to a large extent in the final product. For this purpose the component of the Mixture containing the oxidation-sensitive alloying elements according to the invention to a small particle size be pulverized. This is explained in more detail in the following example.

Example 2

Three powder mixtures were prepared, all of which had the composition 1% chromium, 2% nickel and $ 0.5 molybdenum, the remainder being iron. In all mixtures, the components consisted of water atomized steel powder with 2% nickel and 0.5% molybdenum and ferrochrome powder with 5% chromium and 0.355 carbon. In the first case (CJl -had that

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Ferrochromium powder has an average particle size according to Fischer of 33 μm, in the second case (D) ZOjm. and in the third case (E) Imm. Before compression, 0.5 $ graphite powder and 0.8 $ zinc stearate were mixed into the three powders. These were then pressed together into cylinders, which were heated at 1120 ° C in partially burned propane with a dew point of 3C ₁ and then thermoformed in the manner described in Example 1. The cylinders were cut in half and the cut surfaces ground and polished. The chromium content was determined at different points of the cutting

area measured using a microprobe. The coefficient of variation (CY) for these data was calculated as the standard deviation in percent of the mean concentration. CV is a measure of the heterogeneity of the material. Material made from Powder C had a CV of $ 175. For powder D a GV value of $ 135 and for powder E of k5%. With a heterogeneity corresponding to a CV value of $ 150, the hardenability-improving effect of chromium almost does not exist, ie the addition of chromium in the Powder C has no value, whereas Powder G and particularly Powder E gave a material in which the hardenability was improved by adding chromium.

From the above examples, the advantage of adding this component is evident, "did oxidation-sensitive alloying elements in a 'finely powdered addition-

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stand contains. The powder particles had a mean Fischer size below 20 µm. The example shows

if

also that it is even more advantageous if the middle. Particle size smaller than

Of course, it is important that the powder described above not ooze out after mixing. Sucking out possibilities occur during the transport of the powder from the mixer to the consumer and during the introduction into the powder press. One way to reduce the leakage rate is to add 50 to 200 grams of a light mineral oil per ton of powder continuously while the mixer is being filled. This causes the fine components to stick to the coarser steel particles.

A further improvement in this regard is achieved if the mixture is subjected to a heat treatment at 65O to. .900 ⁰ C for a time of 15min to 2h in a reduced atmosphere with the following precautionary measures in the disintegration of the cake is subjected. As a result of this treatment, the finely pulverized alloy powder particles are sintered onto the steel powder particles, which effectively counteracts seepage. This preventive sintering treatment can also be applied to powders to which oil has been added in the above-mentioned manner

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the same applies to powders to which no oil has been added.

In those cases in which the powder mixture has a low proportion of finely powdered alloy powder, this can preferably be mixed with only part of the steel powder to form a concentrate. This The concentrate is then subjected to one of the measures described above to reduce the risk of leaking out. Finally, the concentrate is mixed with such an amount of steel powder that the desired composition is achieved will.

In addition to the components listed, the Mixtures contain a suitable lubricant, for example zinc stearate. This lubricant additive should do not exceed one percent.

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Claims (8)

Claims 1. Powder mixture for the production of articles made of alloy steel with small and few oxide inclusions, characterized by a metal powder component which consists of a mixture of two powders, namely an atomized pre-alloyed steel powder and a fine pulverized powder containing alloying elements, with alloying elements whose oxides are one Free energy of formation with an absolute value of less than 120 kcal / mol O ₂ (502 kj / mol Og) at 1000 ° C, are essentially contained in the atomized pre-alloyed powder, while all alloy elements whose oxides have a free energy of formation with an absolute value of 120 kcal / Mol Ou (502 kj / Mol O ₂ ) at 1000 ° C Exceeding, possess, are completely contained in the finely pulverized powder. 2. Powder mixture according to claim 1, characterized by 80 to 99% of an atomized pre-alloyed powder of iron and 0.2 to 10 $ of the alloying elements, the oxides of which have a free formation energy kit., An absolute value less than 120 kcal / mol O ₂ (502 kj / mol 0 _g ) at 1000 ^° C 509823/0658 possess, and a finely pulverized powder consisting of a proportion of at least 25 % and preferably 50 % of the alloying elements, the oxides of which have a free energy of formation with an absolute value of 120 kcal / mol Og (502 kJ / mol Og) at 1000 ° C exceeding ,. a maximum of 10 % carbon, the remainder iron and / or other alloying elements, the oxides of which have a free energy of formation with an absolute value lower than 120 kcal / mol O ₂ (502 kJ / mol Og) at 1000 ° C. 3. Powder mixture according to claim 1 or 2, characterized in that the alloying elements in the atomized pre-alloyed powders comprise one or more of the elements nickel, copper, molybdenum and cobalt, If. Powder mixture according to one of Claims 1 to 3, characterized characterized in that the alloying elements in the finely powdered powder are chromium and manganese alone or . included together. 5. Powder mixture according to one of claims 1 to If, characterized in that the finely pulverized powder of a ferroalloy with a total of at least 25 $ and preferably at least 50 pounds of the alloying elements, the oxides of which have a free energy of formation with an absolute value of 120 kcal / mol Og (502 kj / mol Og) at 1000 ° C +/- at least 509823/0658 2Λ55850 owning in excess of $ 10 carbon and at most Remainder consists of iron. 6. Powder mixture according to one of claims 1 to 5 »characterized in that the finely pulverized powder has an average particle size according to Fischer of at most 20 pm and preferably of at most 5 pm . 7. Powder dung according to one of claims 1 to 6, characterized in that the particle size distribution of the atomized pre-alloyed powder is such that more than $ 0% and preferably more than S7% of the powder passes through a sieve with a mesh size of 175 µm. 8. Powder dung according to one of claims 1 to 7, characterized in that, in addition to the metal powder content, it contains graphite and / or a lubricant such as zinc stearate up to a maximum of 1% each. 509823/0658